This invention relates generally to amorphous metal alloys. In particular, the invention relates to cobalt or iron-based amorphous metal alloys having high tensile strength and high electrical resistivity.
An amorphous material has a disordered atomic-scale structure. In contrast, crystalline material has a highly ordered arrangement of atoms. Therefore, amorphous materials are non-crystalline. Amorphous metals are typically an alloy rather than a pure metal. These amorphous metal alloys contain atoms of significantly different sizes, leading to low free volume (and therefore up to orders of magnitude higher viscosity than other metals and alloys) in molten state. The high viscosity prevents the atoms from moving enough to form an ordered lattice. The disordered atomic-scale structure also results in low shrinkage during cooling. The absence of grain boundaries in amorphous materials, the weak spots of crystalline materials, leads to better resistance to corrosion.
Amorphous alloys have a variety of potentially useful properties. In particular, they tend to be stronger than crystalline alloys of similar chemical composition. Amorphous metals derive their strength directly from their non-crystalline structure, which does not have any of the defects (such as dislocations) that limit the strength of crystalline alloys.
Amorphous metal wires of small diameter (e.g., 5-150 microns), also referred to as microwires, can be produced by the Taylor-Ulitovsky production process, in which a glass tube and the desired metal are brought into a high-frequency induction field. The metal is melted, and its heat softens the glass tube, so that a thin metal filled capillary is drawn from the softened glass tube. The metal-filled capillary enters a cooling zone in a superheated state where it is rapidly cooled, such that the desired amorphous structure is obtained. In this process, the alloy melt is rapidly solidified in a softened glass sheath. The presence of the softened glass sheath dampens instability in the alloy melt and promotes the formation of a glass-coated microwire with uniform diameter and a smooth metal-glass interface. Rapid cooling is typically required to obtain amorphous structures. The rate of cooling is not less than 104 degrees C./sec and preferably is 105 to 106 degrees C./sec.
In the past, crystalline metal alloys have been used in radiation detectors. One type of radiation detector is a proportional counter, and this type of detector is often used for neutron detection. A typical proportional counter includes a substantially cylindrical cathode tube, and an anode wire that extends through the cathode tube. The anode wire is very thin, approximately 5-25 microns in diameter, and should have a high electrical resistance. The cathode tube is sealed at both ends, and may be filled with a gas, such as Helium-3 gas. This gas is ionized when irradiated by incident radiation. The anode wire is insulated from the cathode and is typically maintained at a positive voltage while the cathode is at ground.
During use, incident radiation, such as neutrons, interacts with the gas inside the cathode and produces charged particles that ionize the gas atoms and produce electrons. The electrons are drawn to and strike the positive anode wire and create a current pulse that can be detected. The magnitude of the current pulse is proportional to the energy liberated in the ionization event (i.e., a neutron interacting with the ionizable gas).
In some applications proportional counters can be used as position sensitive detectors in which the location of the arriving ionized electron is determined from either the difference in the rise times of current pulses at opposite ends of the wire or from the relative amounts of charge reaching the ends. The spatial resolution of the position sensitive detector is enhanced by increasing the electrical resistance of the anode wire, which slows down the current pulses, increasing the time for the control electronics to detect the current pulses. Accordingly, high resistance anode wires are preferred to improve the spatial detection resolution of position sensitive detectors.
Radiation detectors, proportional radiation counters and neutron detectors are often used in harsh environments. The detectors can be exposed to extreme low and high temperatures, to low or high frequency vibrations and to corrosive environments. Designing a very thin anode wire to survive in these environments can be a challenge. The anode wire preferably should have high electrical resistivity (for good spatial resolution), a smooth surface finish (for uniform resistance over it's length), corrosion resistance (for harsh environments), and high tensile strength (to eliminate deleterious effects due to unwanted vibrations).
The anode wire is placed under tension during assembly of the radiation detector, and the wire must survive the manufacturing process as well as thermal and mechanical stress imparted during service. Crystalline metal alloys used as anode wires have low tensile strength and plastically deform once their tensile strength is exceeded. The failure of the anode wire and/or a change in its dimensions due to plastic deformation degrades the operation of the radiation detector. Additionally, when the radiation detector is used in some applications, it is desirable to render the radiation detector insensitive to low frequency vibrations. Typically, this is achieved by placing the anode wire under high mechanical tension. Unfortunately, crystalline metal alloys experience a high failure rate and a short service life. Accordingly, a need exists in the art for an anode wire that has high electrical resistivity, a smooth surface finish, good corrosion resistance and high tensile strength.